The present invention relates to an antenna device and a portable radio incorporating the antenna device and more particularly aims at reducing the size of a portable radio by simplifying the circuit configuration of an antenna device.
Radiation characteristics during a call in the antenna device of a typical conventional portable radio are strongly affected by the human body (of a user). In such a conventional portable radio, a diversity antenna for switching a plurality of antennas from one to another is used for improving the radiation gain of the antenna.
FIG. 4 shows an antenna device in a conventional portable radio. The antenna device comprises an antenna element 1 projecting outward from the casing 2 of the portable radio, an internal antenna element 8 provided inside the casing 2, a joint 4 which is electrically connected to the feeding point 3 of the antenna element 1, a matching circuit unit 5 for matching the input impedance of the antenna element 1 with the radio circuit unit 6 of the portable radio, a switch 7 for selecting the antenna element 1 or the internal antenna element 8, and a control unit 9 for controlling the switching of the switch 7 by comparing the levels of the electric power received by the antenna element 1 and the internal antenna element 8.
While the switch 7 selects the antenna element 1 in this system, the power received by the antenna element 1 is transmitted to the radio circuit unit 6 via the joint 4 and the matching circuit unit 5. The level of the power thus transmitted to the radio circuit unit 6 is stored in the control unit 9. Then, the control unit 9 sets the switch 7 so as to connect the internal antenna element 8 and the radio circuit unit 6. The power received by the internal antenna element 8 is transmitted to the radio circuit unit 6 and the then power level is stored in the control unit 9.
The control unit 9 compares the levels of the power respectively received by the antenna element 1 and the internal antenna element 8 and changes the switch 7 over to the antenna element that has received the higher power level. The signal transmitted from the radio circuit unit 6 is radiated through the antenna element selected by the control unit 9.
One of the antenna elements is selected in accordance with the level of the power thus received. Therefore, even when the radiation is restrained because one of the antenna elements is situated closer to the human body, the radiation can be given off through the antenna element where the restraint of radiation is lower, whereby the influence of the human body on the radiation characteristics of the antenna can be reduced.
However, there still exists a problem arising from causing the circuitry to be complicated in the conventional antenna device because the switch and the control unit are required to switch antenna elements from one to the other. Another problem lies in the fact that the portable radio can hardly be reduced in size because the number of parts tends to increase.